The IEEE 1584 calculations are based upon an empirically derived model and statistical analysis to produce mathematical equations and provides a reasonable degree of accuracy. The 2018 guide was developed from over 1860 short circuit tests performed at various voltage levels. It is important to understand however, that there are limitations to calculations based upon the standard.
The model is valid for systems having:
- Voltages in the range of 208 V – 15000 V, three-phase.
- Frequencies of 50 or 60 Hz.
Bolted fault current (Prospective Short Circuit Current) in the range of:
- 700 A to 106,000 A for voltages between 208 and 600 volts.
- 200 A to 65,000 A for voltages between 601 and 15,000 volts.
- Working Distances greater than or equal to 305mm.
- Grounding of all types and Ungrounded. (Earthed and Unearthed systems)
- Equipment enclosures of maximum height and width 1244.6mm. The width needs to be larger than four times the gap between the conductors.
Gaps between conductors in the range of:
- 6.35 to 76.2mm for voltages between 208 and 600 volts.
- 19.05 to 254mm for voltages between 601 and 15,000 volts.
- Faults involving three phases only.
- AC (Alternating Current) faults only.
There are some key points to be made when interpreting the results from the arc flash calculations which are summarised as follows:
- Arcing Duration has a linear effect on the incident energy explains why lower prospective short circuit current does not always correlate to low incident energy levels.
- Distance from the arc has an inverse exponential effect meaning that small changes in distance can have large changes in incident energy.
- X/R ratio, supply frequency and electrode material. Variations in X/R ratio of the system, the supply frequency or electrode material do not affect the results. Although the testing was at 60Hz the system of equations for calculating arcing current and incident energy should be accurate over the range of 50 Hz to 60 Hz. Conductor material was not found to be significant although other testing standards do use copper and aluminium electrodes.
- Arcing Current depends primarily on available short-circuit current, bus gap (the distance between conductors at the point of fault), electrode configurations, enclosure size, and system voltage.
- Incident Energy depends primarily on calculated arc current, arcing duration, and working distance. Electrode gap is a smaller factor.
- Earthing. In the IEEE 2002 model there were different factors for earthed (grounded) and unearthed (ungrounded) systems. This has now changed, and the IEEE 1584-2018 model does not have system earthing configuration as an input parameter. The test results did not show any significant impact of the system grounding or bonding on the incident energy released by the arc.
- Arc Sustainability. The commentary to the earlier IEEE 1584 2002 guide stated that it was very difficult to sustain arcs at lower voltages they were only able to sustain an arc once at 208 volts. The 2018 model states that “Sustainable arcs are possible but less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current less than 2000 A”. At lower voltages, there is always the possibility that the arc will self-extinguish but that would be very difficult to model.
- Single Phase Systems. At European single-phase harmonised voltages, (220/230 volts) we do know that severe burns have been caused to operators. However, IEEE 1584 deals with three phase faults only. Although there have been various papers that have suggested applying a correction factor to a three-phase result, I remain sceptical that this would be meaningful or accurate. A conservative approach would be to use the single-phase voltage and prospective short circuit current and then base a prediction of hazard level on a calculation of the three-phase incident energy.
- DC systems are not included in the IEEE 1584 model although there has been a desire at committee level for many years to include DC testing. It is possible that DC testing will be developed in the future.
- Fault Types. All testing used in the basic incident energy model was three-phase testing because three-phase arcs produce the greatest possible arc flash hazard in ac equipment. Open LV Boards and bare conductor lines where single-phase faults are likely can only be addressed as three-phase faults using the models in this guide. Other possibilities are; (i) It is widely recognised that line-to-line faults in equipment or cables often quickly escalate into three-phase faults. (ii) Low voltage system earth faults will also often escalate very quickly into three-phase faults.
- Arcing Current Variation Factor. The model relies upon a calculation of the predicted arcing current value so that the operating time for the upstream protective device can be determined. I explain the fault level paradox later which talks about very large variances in incident energy due to very small changes in the operating time of protective devices due to settings or tolerances. The solution is to make two arcing current and energy calculations; one using the calculated expected arc current and one using a reduced arcing current that is lower. This is achieved by using an arcing current variation factor and then calculating a second incident energy level and arc flash boundary. The hazard severity is therefore, based upon the higher results of energy level and arc flash boundary.
The IEEE 1584 Guide for Performing Arc Flash Hazard Calculations can be purchased from the Institute of Electrical and Electronic Engineers. https://standards.ieee.org